Joost de Valk

Yoast

Allows you to easily find the custom fields (including hidden custom fields) and their values for a post, page or custom 
post type post.
Just go to Tools – Custom field finder, enter the post ID of any post and you’ll see all the custom fields. This includes 
the custom fields that start with an underscore ( _ ) that are normally hidden from the custom fields interface.

Custom field finder

Allows you to easily find the custom fields (including hidden custom fields) and their values for a post, page or custom post type post.

aThemes

vladff

The Sydney Toolbox plugin is meant to be used only with the Sydney WordPress theme.
This plugin registers custom post types and custom fields that are needed in the <a href="https://wordpress.org/themes/sydney/" rel="ugc">Sydney theme</a>

Sydney Toolbox

Registers custom post types and custom fields for the Sydney theme

Custom field finder vs Sydney Toolbox